This invention pertains generally to the storage of thermal energy and in particular to such storage as latent heat of fusion in a ternary eutectic salt mixture.
Storage of thermal energy has gained great importance since the increased interest in the use of solar energy. On account of the fluctuation in electrical usage and the limited availability of the sun during a twenty-four hour period, a practical solar-energy collector requires a large-scale, demand-sensitive energy storage. Otherwise, the solar-power system must be connected to an electrical-power grid, which accomodates the changing input by means of peaking units. Such units are usually fast-response, fuel-fired power generators that are generally of lower efficiency than base-load units.
One approach to large-scale, demand-responsive storage proceeds by introducing thermal energy into one or more quantities of a thermal-energy storage material by means of an energy transfer medium. In operation, a system utilizing this approach has hot fluid passing through pipes which are submerged in a liquid energy-transfer medium. The medium boils and the vapors are condensed on the sides of the one or more containers filled with a thermal energy-storage material. The heat of condensation causes the storage material to melt, thereby storing the thermal energy in the molten storage material.
Presently several techniques are known for providing large scale demand-responsive energy storage. Most of these techniques are not suited for regional energy production, but rather for a few buildings at best. A technique suited for regional energy production employs a mixture of hydroxides. The disadvantages of using hydroxides are that these compounds are corrosive and dangerous and do not occur naturally. Another technique which is promising for regional energy production utilizes mixtures of carbonates. Carbonates unfortunately have a melting point too high to be compatible with almost all organic heat transfer media. For a material to be acceptable for thermal energy storage, the melting temperature can be neither too high (e.g., carbonates) nor to low. If the melting point is too low, high-pressure super-heated steam cannot be used and the efficiency of the whole system is low. An example of a thermal energy storage material having a melting temperature which is too low is clathrates. It is also important the storage material is reasonably priced and available. Fluoride compounds fail these requirements. Although some of these compounds have excellent properties, these fluorides, like all fluorides are expensive.